This invention relates to a camera using a film having a magnetic recording portion, for detecting an unexposed frame by detecting the presence or absence of magnetic information recorded on the film when a film cartridge having some frames photographed is loaded.
Conventionally, film cartridge type camera systems in which films can be easily replaced are proposed. In a camera using a cartridge film having a magnetic recording area, the amplitude of a reproduction signal of magnetic information recorded on the film is normally proportional to the movement speed of the film. However, the film movement speed at the time of feeding of the film in the camera is unstable in some cases, and therefore, the amplitude of the reproduction signal is not stable.
Since a magnetic layer formed on the film is formed to be substantially transparent so as not to cause any problem in photographing, the amplitude of the reproduction signal is not so large. Further, since a motor mounted on the camera main body generates noises harmful to a circuit in the main body at the time of driving, it is difficult to reproduce information magnetically recorded on the film with a high S/N ratio.
Therefore, various conventional techniques are provided to stably detect the reproduction signal by changing the gain of the amplifier in the reproducing circuit, changing the characteristic of the filter to eliminate the noise, or changing the film feeding speed according to the conditions of reproduction.
The technique for measuring the level of a noise signal contained in the reproduction signal and determining a determination level used for determining the presence or absence of magnetic information based on the noise level is disclosed in Jpn. Pat. KOKAI Publication No. 9-211662. By use of this method, it is possible to detect the magnetic information even if the S/N ratio is low to some extent.
The noise generated from the camera can be dealt with by shielding the motor or properly changing the design of the reproducing circuit. However, when the condition in which the user uses the camera is taken into consideration, it cannot be said that a perfect method is provided.
Recently, in an environment in which the camera is used, various electronic devices for generating electromagnetic noises, for example, a personal computer, portable telephone, television, radio device are used. The user may replace film cartridges irrespective of the environment of electromagnetic noises.
However, in the environment in which the electromagnetic noises are generated, magnetic information cannot be successfully detected in some cases. That is, in such a case (in the environment in which the electromagnetic noises are generated), there occur problems that a frame of double exposure may be provided and a frame or some frames which are unexposed may be provided in the successive exposed frames.